This invention relates to a semiconductor integrated circuit device having circuits for protecting the input terminals of a MOS IC against an external electrostatic surge.
In recent years, the IC packing density of a semiconductor device has been increased. The gate oxide film of the device has been made thinner and thinner. Today semiconductor devices whose gate oxide films are 400 to 500 .ANG. are commercially available. Such a thin gate oxide film may be broken if an electrostatic surge voltage is applied to it from a human body, for example. When the gate oxide film is broken, the MOS transistors will be short-circuited.
Measures against such a short-circuiting are disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 54-116887. The semiconductor device shown in this publication includes a protective circuit for protecting an input MOS transistor against an external surge. The protective circuit is provided between an input terminal and the gate of the input MOS transistor. It comprises a first resistor connected at one end to the input terminal, a MOS transistor having its drain connected to the other end of the first resistor, and a second resistor connected between the source of the MOS transistor and a reference voltage source or a semiconductor substrate. The gate of the MOS transistor is connected to the reference voltage source or the semiconductor substrate. An external surge is discharged through the protective circuit, and the input MOS transistor is thus protected.
The drain potential of the MOS transistor of the protective circuit is equal to the input voltage divided by the sum of the resistances of the first and second resistors. Nonetheless, a considerably high voltage is still applied on the gate of the input MOS transistor.